Mechanical dynamometers are often used for static and dynamic measurings of mechanical forces. The forces to be measured cause changes in the electrical charge, voltage, current or impedance in one or more measuring elements.
In a known accelerometer an acceleration of a seismic mass results in a mechanical force proportional to the acceleration. This force influences one or more measuring elements, such as tension or pressure-sensitive electrical resistances, semiconductors or piezoelectric elements, electrodynamic devices or other devices converting mechanical force into electrical energy.
These forces can imply a pressure, a tension or a displacement onto the measuring elements, and it has been found that piezoelectric accelerometers of the shear type can be structured to obtain advantageous properties, such as good linearity, low static and dynamic temperature sensitivity, low sensitivity to magnetic fields and a wide frequency range.
The known piezoelectric accelerometers comprise a base with an upright extending from the base. Piezoelectric elements and seismic masses are arranged on or around the upright.
Examples of piezoelectric accelerometers are
1) accelerometers of the compression type, PA1 2) accelerometers of the "Ring shear" type, cf. U.S. Pat. No. 3,104,334, PA1 3) accelerometers of the "Conical ring shear" type, cf. GB-PS No. 1,507,251, PA1 4) accelerometers of the "Delta Shear" type, cf. DK-PS No. 131,401, and PA1 5) accelerometers of the "Planar Shear" type, cf. DK-PS No. 138,768. PA1 1. a lower, static temperature sensitivity PA1 2. a lower dynamic temperature sensitivity (sensitivity to temperature shocks) PA1 3. a lower sensitivity to magnetic fields PA1 4. an improved linearity PA1 5. a larger frequency range PA1 6. a higher transverse resonance frequency, and PA1 7. lower production costs.
In an accelerometer of the "Ring shear" type, a base with a cylindrical upright is used, around which a cylindrical piezoelectric element is arranged, such as glued, said element comprising a cylindrical seismic mass secured to the outer cylindrical surface of the element. This rotational-symmetrical embodiment has inter alia the result that the elements of the accelerometer can be manufactured in an inexpensive and accurate manner by way of turning and grinding, and that the sensitivity of the accelerometer towards acceleration in a plane perpendicular to the axis is low. This embodiment is, however, encumbered with the drawback that it is difficult to obtain a sufficient strength and thermal stability in the connection between the cylindrical elements, and that the different temperature expansion coefficients of said elements cause undesired mechanical stresses which may affect the measuring results.
An accelerometer of the "Planar Shear" type employs two plane piezoelectric elements secured on the opposing broad sides of a prismatic upright of a rectangular cross section. The elements are secured to the upright by means of a clamping ring, whereby problems of different temperature expansion coefficients are avoided. This accelerometer generates, however, different transverse resonances in different directions, perpendicular to the axis of the prism as said resonances may be lower than the resonances of the cylindrical embodiment.
An accelerometer of the "Delta Shear" type employs a prism with a plurality of measuring elements arranged on the sides of said prism. A shear deformation is caused by the forces to be measured. The prism is of a cross section in form of a regular polygon, a plurality of identical piezoelectric elements being arranged on the sides of the polygon and retained with the seismic masses on the surfaces of said prism by means of a clamping ring. The resulting transverse resonance is substantially the same in all directions. This embodiment is, however, encumbered with the drawback that the upright is difficult to manufacture and join to the base when all the dimensions are small.
A fourth embodiment comprises a base provided with a cylindrical upright with a cylindrical opening. A cylindrical piezoelectric element is arranged in this opening and comprises in turn a cylindrical opening in which the seismic mass is mounted, cf. PCT/SU/00272. This embodiment is encumbered with the drawback that it is difficult to guide the signal conduit to the seismic mass, to which it is to be secured. Another drawback is the securing of the piezoelectric element to the inner side of the cylindrical upright as well as the securing of the seismic mass to the piezoelectric element. These operations are difficult to perform without the use of conductive glue or another binder, whereby the temperature range in which the accelerometer can be used is considerably reduced.